Signal receiving and reproducing system



, June 5- R. M. SPRAGUE SIGNAL RECEIVING AND REPRQDUCING SYSTEM I H Fr yEN i mm INVENTOR /UM I A OR Y June 12, 1945. v R. M. SPRAGUE SIGNALRECEIVING AND REPRODUQING SYSTEM Filed Sept. 26, 1942 3 Sheets-Sheet 2fiqasxr/l d'px/muc INVENTOR ATTORNE 5 muciuwm km June 1945. R. M.SPRAGUE SIGNAL RECEIVING AND REPRODUCING SYSTEM Filed Sept. 26, 1942 3Sheets-Sheet 3 INVENTOR ATTORNEY Patented June 12, 1945 SIGNAL RECEIVINGAND REPRODUCIN SYSTEM 7 Robert M; Sprague, Hicksville, N. Y., assignorto Press Wireless, Inc., C

- of Delaware hicago, 111., a corporation original'application October27, 1939, Serial No. 301,563. Divided and this application September 26,1942. Serial No. 459.759

13 Claims.

This invention relates to wave signalling systems and more especially tosystems of carrier wave transmission wherein the effects of parasitic's,noise, harmonic distortion fading and undesirable level variations areto be avoided.

A principal object of the invention relates to a system for transmittingsignals whether oral, visual or control, through the intermediary of atransmission link, for example a high-frequency channel or radiochannel, wherein the signal reproduction is rendered substantiallyindependent of undesirable level changes. 7

Another principal object is to provide an improved radio signallingsystem which is singularly free from the effects of fading, echo,harmonic distortion. parasitic noises and the like. I

A further object is to provide an improved manner of receiving messagesignals through the intermediary of a frequency modulated subcarrier.

A further feature relates to a receiver or reproducer of facsimilesignals and the like, employing a specially designed band-pass filterfor eliminating harmonic distortion in the transmitting medium or linkand alsofor eliminating noise components below the actual frequencyspectrum employed for signalling.

A further feature relates to a facsimile receiver or the like for use ina frequency modulation system and having an automatic level controldetermined by the received side-bands as distin-' guished from thecarrier. v

A further feature relates to a receiver for frequency modulation systemsemploying the combination of a specially designedfilter and powereration of the following detailed'descriptions and the appended claims.I

' While the invention will be illustrated in connection with facsimiletransmitting and receiving systems, it will be understood that'this ismerely for explanatory purposes and that various features and aspects ofthe invention can be employed in other kinds of signalling or controlsystems. Accordingly in the drawings,

Figs. 1 and 2am a schematic wiring diagram of a receiving systemaccording to the invention. Fig. 3 is ,an enlarged diagramof part of theconverter unit of Fig. 2. I

Figs. 4 and 5 are respective modifications of the converter sectionillustrated in Fig. 3.

In the drawings (Fig. l), the block T represents a transmitting sourceof the type described in application Serial No. 301,563, filed October'27, 1939, (Patent No. 2,299,937, granted October 2'7, 1942) whereby theoriginal signals e. g, lights and shades of picture or the like areconverted into a subcarrier of audio frequency which carrier isfrequency modulated in accordance with the picture ranges orthe like.

quency modulations is so arranged that the uppermost frequency e. g.3000 C. P. S. is less than supply an undistorted signal ofunvaryinglevel to ture definition is obtained-at higher transmission speeds andwherein the ratio of signal-to-noise is high and the susceptibility toselective fading is decreased. I

Other features and advantages not specifical ly enumerated will beapparent after a considthe second harmonic of the lowermost frequency e.g. 1800 C. P. S the limits of said spectrum representing certain extremeshades of the picture or other subject matter being transmitted. Thisspectrum-limited frequency modulated subcarrier is then transmitted overany wire line or over a radio channel by conventional amplitudemodulation arrangements. i p

The transmitted signal from source T, e. g. in the form of an amplitudemodulated radio carrier wave, is applied to any well-known form of radioreceiver represented by the block RR, wherein the modulations of from1800 to 3000 cycles are detected or demodulated. These demodulatedsignals are then applied to the resistor pad comprising resistorelements 20l to'205 passed by the corresponding filter at thetransmitter. Filter 206 also eliminates any noise voltage outsideitsband-pass. Since there may beside-band frequencies present because ofthe variation ofthe carrier during transmission, the lower cutofffrequency of filter 206 must not be In accordance with said application,the spectrum of the freis connected to the suppressor-grid 2360,

chosen too close to the lower end of the actual signal band, that is,too close to 1800 C. P. S. nor to the upper limit of this band, that is,3000 C. P. S. Preferably, the lower cutoff of filter 206 is spaced fromthe lower limit of the signal band the same distance as the spacingbetween the upper cutoff of filter 20B and the upper limit of thesignal'frequency band. Thus the pass-band is symmetrical and issufficient to pass all desired side-band frequencies. The filter used inpractice passes between 1500 and 3300 cycles.

The signal from filter 206 may vary in amplitude and must therefore beironed out to a uniform level. This is effected by the A. V. C. circuitof amplifier tubes 236 and 25l and by the power limiter tube 21!. Tube211 is a straight amplifier tube preferably of the double triode typehaving two separate output circuits, one of these output circuitsincluding plate 2l1a and cathode 2| 1?) leads to the monitoring meter226 and monitoring jack 345, whereby the average level of the signal canbe determined and adjusted by potentiometer 213,. The other outputcircuit including plate 2110 and cathode 211d feeds into amplifier tubes236 and 25!. Tubes 236 and 251 are preferably high gain pentodes havinga remote cutoff. The D. C. plate supply for tube 251 is derived from asuitable A. C. source 342 over power transformer 333, full-waverectifier 334, D. C. smoothing filter 335, 336, 331, conductor G, thencethrough the low frequency choke 280, conductor GI and resistor 233 toplate 2l1c. Plate supply conductor GI is also connected by way ofresistor 221 and 222 to plate 2l1a the various resistors being providedwith suitable by-pass condensers as shown. The signal developed acrosscoupling resistor 229 is impressed through condenser 230 on thecontrolgrid 236a of tube 235, the cathode 23612 of which The plate 236dand screen-grid 235e are supplied with appropriate D. C. potentials fromconductors G and G1. The output of tube 236 is fed to a similaramplifier pentode 25!. The output of tube 25! is coupled throughresistor 255 and condenser 256 to the connected control grids 264a,2641) of the double triode tube 254. The signal current in the circuitof plate 2640 is applied to the rectifier tube 252 and this rectifiedsignal voltage is filtered and smoothed by the resistance-condenserfilter 248, 249, 250. The rectified Voltage is then applied over A. V.C. conductor H through resistor 241 and 235 to the respective controlgrids of tubes 25I and 236. Thus when a drop in level occurs, thevoltage rectified by tube 252 decreases, with a similar decrease innegative grid bias of tubes 236 and 25l and the gain is therebyincreased to bring the voltage back approximately to its formermagnitude. In this A. V. C. arrangement, the A. V. C. signal istherefore controlled not by the level ofthe received radio carriervoltage but by the demodulated signal voltage. Thus, even though thecarrier remains constant and the side-band signals fade, the demodulatedsignal will remain constant.

The other plate circuit of tube 264 feeds the power limiter tube 21!.Tube 21I performs several functions. First the action of the A. V. C.circuit above described is not instantaneous because of theresistance-capacity filter and therefore on a sudden fade of thereceived radio signal, the latter will drop for a short period.Secondly, the A. V. C. is not perfect and should the signal fade to theorder of 30 db., the A. V. C. will hold the fade down to only 3 or 4 db.I have found that this level variation is still too great for the bestresults in facsimile or picture transmission. Tube 21I takes care ofboth these conditions by amplifying the signal impressed thereon bytransformer 269 up to a predetermined value and then as the signalincreases further the output remains constant over a very wide range.The tube 211 is preferably connected in balanced or push- .pull relationto the secondary winding of transformer 269, and the control-grids 21la,21lb are at zero bias with'respect to the associated cathodes 21lc,211d. Consequently, as the grids swing positively, grid current fiowsthrough the resistance 210. This automatically applies a bias to thetube 21! which bias builds up substantially in proportion to the inputvoltage, thus holding the output substantially constant. Preferab1y,.aresistance 213 is provided in the common plate lead to make thecharacteristics of the tube 21! linear up to a certain point and then tocause it suddenly to flatten out. This is probably due to the factthatas the grid voltage increases positively, the bias builtup inresistor 21!! does not increase by quite the same amount, but resistor213 serves to lower the plate voltage as the plate current increases.The output of tube 21I is a substantially square wave whereinsubstantially only odd harmonics are present as a result of the balancedor push-pull connection. These odd harmonics must be eliminated beforepass ing the signal to the converter circuit, the elimination beingeffected by filter 214 which is a low-pass or preferably a band-passfilter, the lower cutoff portion eliminating any noise introduced at anypreceding point in the circuit. Filter 214 may be identical to filter206.

The converter The signal is then applied to the converter circuitwherein the frequency changes in the signal are converted to amplitudechanges. The control elements of this converter consist of resistor 28l,condenser 282 and inductance 283 connected to the plate circuit ofamplifier tube 216. The condenser 282 and inductance 283 are designed soas to be series resonant at some frequency below the lowest signalfrequency, which has been assumed to be 1300 cycles. In order to makethe resultant voltage applied to the control grid of amplifier tube 281linear, with respect to frequency, the impedance of condenser282-inductance 283 combination must be low compared to resistance 28!even at the highest frequency passed. The series resonant frequency of282 and 283 is placed at the lower frequency attenuation peak ofband-pass filter 214, for example at ap proximately 1380 C. P. S. Thisis don because at frequencies less than the said series resonantfrequencies, the voltage will again increase and will reach a maximum atvery low frequencies, with the result that any noise being received atfrequencies less than the said resonance frequency will be transmittedat greater amplitude than the frequency adjacent the resonant fre quencyof the 282 283 combination, Consequently, it is necessary to place theresonance frequency at some value at which all noise has been filteredout. Of course, the nearer the said resonance frequency is to the lowestsignal frequency being transmitted e. g. 1800 C. P. the greater will bethe change of voltage with change of frequency, thus giving a betterresponse ratio of signal-to-noise. Another resistor 284 is providedhaving about one-tenth of the value of resistor 281 and it serves toprevent low frequency noise that the filter 214 may not have completelyeliminated, from rising to too great a value. This resistor 284 mustalso be high compared to the maximum impedance of 282-483, otherwise anundesirable phase shift will result. The converted signal. isthenapplied. to an amplifier tube 281 which may be of any'suitable typepreferably a type 6C8 upon whose main control grid 281a the convertedsignal is impressed. The output of tube 281 is further coupled inbalanced relation to another amplifier stage comprising tubes 30| and302 connected in push-pull relation whereby the signal level is raisedtothe desired value. It will be understood of course 'that any well-knownlinear amplifier may be used for amplifying the converted signal.Theamplified output of tubes 30], 302, is then rectified in asuitablerectifier 224 and the rectified output is passed through alow-pass filter 304. This filter attenuates all frequencies above thelowest transmission frequency e. g. 1800 C. P, S. leaving only thedemodulated varying amplitude signal corresponding to the originalphotoelectric cell signal as delivered at the machine facsimile atsource T- The variable amplitude signal from filter 304 can be applieddirectly to any well-known picture reproducing machine.

Y However, in the event that the machine R is of a type which isoperated on a modulated audio frequency, for example an 1800 cycle tone,the signal from filter 304 is used to modulate a locally generated 1800cycle source. For this purpose, there is an oscillator tube 301 which isprovided with circuit cnnections308, 309, 3l0, 3'! l, whereby asustained. 1800 cycle tone is generated. This 1800 cycle tone or currentis then impressed in balanced relation on the controlgrids 3|8a, 3l0b ofa modulator tube 3I8. The plates 318e, 3l8d, are supplied with platepotential from the output of filter 304 which is connected in balancedrelation .to the primary of coupling transformer 320. As a result, thereis impressed upon the balanced amplifier tube 322, an 1800 cycle currentmodulated in accordance with the signals from filter 304. If desired,the

"output of the amplifier of tube 322 may be passed through a filter 326having a band-pass width suitablefor passing the 1800 cycle wave withthenecessary side-bands. The modulated-1800 cycle signal may then'be passedthrough ,a suitable resistor pad 321 to 33l and thence to the facsimilereproducing machine R. It will be under stood of course that thefacsimile transmitting machine at source ,T' and the facsimilereproducing machine R are synchronized by any method well-known in thefacsimile art- In order that the proper range. of tone values may beemployed at the receiver RR, means are provided for obtaining zerooutput of the locally generated 1800 cycle carrier from tube 301 tocorrespond with zero voltage on the control-grid of the frequencymodulator tube at the transmitter. This is accomplished by applying afixed negative bias on the plates 3l8c, 3l8d of the ageof zero isappliedto the plates of tube 3! which is the threshold point or zerovoltage point. Under the above assumption of a minimum signal frequencyof 1800 cycles, this zero voltage corresponds therefore to this lowersignal frequency, consequently any change in the frequency applied totube 216 will give a corresponding 1800 cycle output from the modulator3l0.

In adjusting the receiver, the average input is adjusted to normal bythe meter 226 and the level of the received signal is increased bypotentiometer 215 until the voltmeter 306 reads zero, 4

thus placing the balanced modulator 3l8 on the threshold of operation.

The apparatus at the transmitter having been adjusted as described insaid U. S. Patent No. 2,299,937 so as to provide the requisite frequencyshift of 1200 C(P. S. i. e. from 1800 to 3000 C. P. S. for the range oftone values to be transmitted, this change of frequency will cause atthe receiver a voltage to be impressed on the plate of tube3l8-resulting in a certain level of the local 1800 cycle oscillationsapplied to tube 322. This level is adjusted to normal by increasing thegain by means of potentiometer 32!. After these preliminary adjustmentsatthe transmitter and receiver, the entire system is in readiness forthe transmission of the subject matter which may be a picture, writtenmatter or any visual display consisting of variations of tone or shadevalues.

While in the foregoing description reference has been made to thetransmission of a picture by means of frequency modulations between 1800C. P. S. and 3000 C. P.- S.-, itwill be understood that the invention isnot necessarily limited thereto. However, I have found from actual testsand demonstrations that the above shift of from 1300 to 3000 C. P. S. ismore than sufficient to transmit all the frequencies present in modernpicture or facsimile transmitters. On the other hand, I have transmittedwith good results with the equipment described, using afrequency shiftof from 1200 to 1800 cycles. Furthermore, while particular apparatus andparts havebeen described, it will be understood that variations may bemade therein as will be obvious to those skilled in the art and withoutdeparting from the spirit and scope of the invention. With the equipmentas described, I have succeeded in passing linearly,

frequencies up to 1400 cycles with no apparent 1 attenuation at theupper frequencies and fur thermore I have transm'ittedarticular speechand music through the equipment mereby by replacing the facsimiletransmitter at source T by a voice frequency generator and by replacingthe reproducer R by a voice frequency reproducer. With a higher-range offrequencies such as between 5000 and 7500 C. P. S., or even 5000 and6500 C, P. S., frequencies up to 4500 C. P. S. can

be passed. The invention is not limited to facsimile transmission or toordinary voice frequency telephony. The system is well suited for secretradio transmission because the original voice signals that may bereceived from source T are radiated from the radio transmitter in anunintelligible form, and in order that they may be reproduced, areceiver such as shown in the drawings and adjusted and correlatedwiththe transmitter T, must be employed. I have also found that a systemas described when used in facsimile or voice communication eliminates toa substantial extent, well-known echo effects. As a result of using thesystem as disclosed, the following among other results have beenobtained.

1. The elimination of all harmonic distortion by using the variouslow-pass and band-pass filters such as 206 and 214 and the eliminationof all noise below the actual frequency range used for signalling.

2. The automatic volume control. is rendered more effectiveby beingcontrolled by the demodulated signal in tube 25l giving constantamplitude to the converted signal applied to tube 216.

3. The supplementing of the automatic volume control by the limiter tube21! and filter 214 whereby all changes in level whichescape theautomatic volume control circuit are ironed out, thus supplying anundistorted signal at perfectly constant level to the succeedingcircuit.

4. The provision of a converter circuit associated with tube 216 wherebyfrequency changes are made to amplitude changes with perfect linearityand with high ratio of signal-to-noise.

It will be understood of course that while one particular form of levelcontrol has been disclosed in connection with the tubes 238 and 25!, anyother well-known form may be employed and in addition if desired,another A. V. C. signal may be derived from the radio carrier and ifdesired, the receiver RR may be of the diversity type. The A. V. C.voltage derived as shown may be applied to control the gain of thereceiver itself, being applied to all controlled tubes in the receiver,or to some of them, leaving the radiocarrier-derived A. V. C. on theothers.

Instead of employing a series condenser-in ductance combination 282, 283(such as shown in Fig. 2 and in the corresponding enlarged view of Fig.3) for inverting the frequency modulations to amplitude variations, anarrangement such as shown in Fig. 4 may be employed wherein the partscorresponding to those of Figs. 2 and 3 bear the same numerals. In thismodification, the condenser 219 is designed to be of high impedancecompared to resistance 284. The voltages developed across resistance 284as a result of the frequencies impressed on tube 216, are used tocontrol the amplitude in the output of tube 281. Instead of using aseries resistancecondenser, a series resistance-inductance combinationmay be employed, this modification being shown in Fig. 5 wherein theimpedance of inductance 283 is low compared to resistance 28!. While theembodiment of Figs. 4 and 5 each give zero voltage at zero frequencywhen operating between 1800 and 3000 C. P. S., only approximately 67percent voltage change is produced; whereas with the series resonantcircuit 282, 233 of Fig.2, when this combination is resonant at 1400 C.P. 8., over the same frequency range of 1300 to 3000 C. P. 8., there isobtained a 300 percent voltage change. Various changes and modificationsmay be made in the disclosure without departing from the spirit andscope of the invention. Thus, if under certain conditions of operation,for example at very high speeds of transmission, undesirable phaseshifts occur in the various filters described, these phase shifts may becompensated for by connecting in circuit with one or more of the filterssuitable phase equalizers. However, it has been found that at ordinarycommercial transmission speeds, such equalizers are not necessary.

This application is a division of application Serial No. 801,563, filedOctober 2'7, 1939, issued as U. S. Patent No. 2,299,937, granted October27, 1942.

I What I claim is:

l. A receiver for receiving a frequency modulated subcarrier transmittedas an amplitude modulated main carrier and wherein the frequencyspectrum of the subcarrier band is limited so that the uppermostfrequency is less than twice the frequency of the lowermost frequency,

' said receiver comprising means to detect the sub-' carrier modulationsfrom said main carrier, at band-pass filter for passing said spectrumwhile eliminating any harmonics of the spectrum frequencies which mayhave been introduced during transmission, means to convert the frequencyband passed by said filter into corresponding amplitude variations, anda signal reproducer operated under control of said amplitude variations.

2. A receiver according to claim 1 in which said frequency spectrum aspassed by said filter is confined to the audio frequency band.

3. A receiver according to claim 1 in which a grid-controlled vacuumtube amplifier is provided for amplifying the detected subcarriersignals passed by said filter, and an automatic volume control circuitis provided for said vacuum tube amplifier said circuit applying a gaincontrol voltage to said amplifier which is determined by the level ofsaid detected subcarrier.

4. A receiver according to claim 1 in which a grid-controlled amplifieris provided for amplifying the detected subcarrier signals passed bysaid filter, an automatic volume control circuit is provided for saidamplifier for applying a gain control voltage to the amplifier which isdetermined by the level of said detected subcarrier, and a power limitertube is provided for reducing the effect of the level variations to avalue less than that obtained by said automatic volume control circuit,and another band-pass filter upon which the output of the power limiteris impressed, said other filter having substantially the same bandpassas said first-mentioned filter.

5. A receiver according to claim 1 in which said converter meansincludes a network which is series resonant at a frequency below thelower limit of said spectrum.

6. A receiver according to claim 1 in which said converter meansincludes a network comprising a resistance, capacitance and inductance,the series resonance of the condenser and inductance combination beingat a point below the lower limit of said spectrum, and the impedance ofthe condenser-inductance combination being sufficiently low comparedwith said resistance so that the resultant voltage amplitude from theconverter is linear with respect to frequency.

7. A receiver according to claim 1 in which a rectifier is provided forrectifying the signals from said converter means and a filter isprovided upon which the rectified signals are impressed, said filtersubstantially attenuating all frequency components below the lower limitof said spectrum.

8. A signal receiver for use in frequency-modulated signal systems,comprising a detector for detecting said frequency modulations in apredetermined audio frequency signal band, a bandpass filter forsuppressing most frequencies below the lowest detected frequencycorresponding to the original signals and for suppressing frequencies ofany harmonics of any frequency in said predetermined band, means toconvert said detected frequencies into waves of substantially uniformamplitude, and means to reconvert said uniform amplitude waves intoother corresponding varying amplitude waves.

9. A frequency demodulator for converting frequency modulations intocorresponding amplitude modulations confined to a predetermined bandcomprising a grid-controlled vacuum tube, a signal input circuitconnected to the control grid of said tube and on which the frequencymodulations are impressed, said tube having an output circuit comprisinga network having a resonant section which has a series resonance at afrequency below the lowest of the frequency modulations impressed onsaid input circuit.

10. A frequency demodulator according to claim 9 in which a band filterisconnected to said input circuit to limit the frequency band applied tosaid demodulator, and said resonant section has a,

series resonance below but relatively close to the lowest of theimpressed frequency modulations, and to said lower attenuation peak ofsaid filter,

11. A frequency demodulator according to claim 9 in which said networkcomprises in series a resistance, a condenser and an inductance, the

combined impedance of the condenser and the resistance, and being highcompared to the combined impedance of said condenser and inductance atthe highest frequency of said impressed signal frequency modulations.

13. A signal receiving system for receiving a frequency-modulatedcarrier wherein the frequency spectrum of the modulation signals has afrequency range within the same frequency range as the frequency rangeof the original signals to be transmitted, detector means to detect saidmodulation frequencies, a network upon which the detected modulationfrequencies are impressed for eliminating all harmonics of said spectrumfrequencies which harmonics may have been undesirably introduced in thetransmission channel to which the receiving system is connected, meansto convert said modulation frequencies from said network intocorresponding amplitude modulations, a separate local source ofoscillations, means to apply said amplitude modulations to said localsource, and a signalreproducing device controlled by the output of saidmodulated local source of oscillations.

ROBERT M. SPRAGUE.

